\documentclass{article}


%%-----------PGF/TIKZ
\usepackage{pgf}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,positioning}

\usepackage{palatino}

% Use preview.sty to crop the page
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{pgfpicture}
\setlength\PreviewBorder{0pt}

\definecolor[named]{Yellow}{cmyk}{.01,.11,.74,0}
\definecolor[named]{Red}{cmyk}{0,.48,.62,0}
\definecolor[named]{Green}{cmyk}{.31,0,.69,0}
\definecolor[named]{Teal}{cmyk}{.39,.05,.18,0}
\definecolor[named]{Purple}{cmyk}{.16,.31,0,0}
\definecolor[named]{Gray}{cmyk}{0,0,0,.29}

\begin{document}
\begin{figure}
\begin{tikzpicture}
    [
    node distance=10mm and 10mm,
    scale=.8,
    text width=5em,
    text centered,
    rectangle,
    rounded corners,
    em/.style ={
        fill=Yellow,
        draw=black,
		thick, 
    },
    brackish/.style={
        fill=Red, 
        draw=black,
		thick,
    },
    ox/.style={
        fill=Green,
        draw=black,
		thick, 
    },
    contact/.style={
        fill=Purple,
        draw=black,
		thick, 
    },
    tm/.style={
        fill=Teal, 
        draw=black,
		thick,
    },    
    txt/.style ={
		text width=5em,
		text centered
    },
    pump/.style ={
        draw,
        thick,
        double distance=1pt,
        -latex'
    },
    gravity/.style ={
        draw,
        thick,
        -stealth'
    },
    ]

	\node(pump) [txt] {From Secondary Treatment};
    \node(contact) [contact,right=of pump] {Contact Basin};
    \node(allen) [em,below=of contact] {Allen};
    \node(gearheart) [em,right=of allen] {Gearheart};
    \node(hauser) [em,right=of contact] {Hauser};
    \node(brackish) [brackish,right=of hauser] {Brackish Marsh};
    \node(bay) [txt,text width=7em,below=of brackish] {Bay Discharge};
    
    \path[gravity] (pump) edge (contact);
    \path[gravity] (contact) edge (allen);
    \path[gravity] (allen) edge (gearheart);
	\path[gravity] (gearheart) edge (brackish);
	\path[gravity] (brackish) edge (bay);
		
\end{tikzpicture}



\end{figure}
\end{document}